During HIV infection there is a profound and selective decrease in CD4 T cells which is associated with progressive immunodeficiency. The mechanism(s) by which HIV infection lead to CD4 T cell depletion are debated. Although recent data gives evidence for a higher viral load in the circulation and in lymphoid organs that had been previously appreciated, even at early stages of disease, recent data from a model reconstituting the SCID mouse with human lymphoid cells also argue that direct viral infection and cytopathicity cannot account for all of the CD4 T cell death. In addition, the fact that chimpanzee can sustain a chronic infection with HIV in vivo, and c cytopathic infection in vitro, but does not progress to acquired immune deficiency syndrome (AIDS), suggests that mechanisms other that direct viral destruction contribute to CD4 T cell loss. We have demonstrated that crosslinking of CD4 on human CD4 T cells followed by signaling through the T cell receptor for antigen results in activation-induced cell death by apoptosis. These results suggest a mechanism for the massive CD4 T cell depletion in AIDS, particularly in the face of concurrent infection and antigenic challenge with other organisms. Evidence for this mechanism of T cell death in HIV infection has come from several groups, including our own, who have shown that CD4 (and in some cases CD8) T cells from HIV-infected individuals undergo apoptosis upon activation in culture. The experiments in this proposal are designed to investigate both the mechanism and role of apoptosis in AIDS. In Aim 1, we propose to confirm our in vitro observations in vivo, and to determine if activation-induced apoptosis plays a role in progression to AIDS. Our in vivo studies will make use of HIV- or SIV-infected tissue from humans and from nonhuman primates to identify apoptotic cells in situ and to determine whether more cells are apoptotic than are productively infected with virus. Progression will be studied by longitudinal and cross-sectional studies of patient cells and serum in order to correlate apoptosis with disease stage and by analysis of apoptosis in primate models which do or do not progress to AIDS-like disease. In Aim 2, we will investigate the mechanism of CD4 priming for apoptosis in AIDS. We will identify the means by which the apoptotic priming signal is delivered to the CD4 T cell in HIV infection, the cell which is susceptible to this signal, the kinetics and propagation of this signal, and the role of specific kinases and phosphatases in the priming event. These experiments will employ PBL's from HIV-infected and uninfected adults and children, lymph node tissue from infected and uninfected humans and nonhuman primates, T cell lines, and human CD4 transgenic mice in conjunction with a number of experimental methods including flow cytometry and biochemical analysis of DNA fragmentation, and protein kinase and phosphatase activation. The proposed studies will investigate a range of aspects of apoptosis in AIDS, from its clinical relevance to molecular mechanisms. Our ultimate goal is the development of therapeutic interventions for HIV-infected individuals, in order to prevent progression to AIDS. Ideally, this would allow anti-viral therapies to eliminate viral infection, while the disease is held at bay.